To make things easier, this is using Crane to orchestrate the Docker containers.
The environment consists of 3 containers: the sample app (with PHP and Composer installed), a server (Nginx + php-fpm), and a MySQL database (accessible from your app via DB_PORT; see crane.json for user/password). The app and server containers are based on michaelsauter/php-base.
Docker and Crane. If you're on OS X and have no Docker environment setup yet, I recommend docker-osx. boot2docker is nice, but unfortunately, it does not support bind-mounting volumes yet so it can't be used for this example.
git clone git@github.com:michaelsauter/silex-crane-env.gitcd silex-crane-envcrane lift
That will build the images and run the containers. Note that on the first run, the server container will need to install the dependencies, so even after the containers are started, the app cannot be accessed until composer install finishes. Use docker logs -f silex_server to see the progress. After the autoload files are generated, try localhost/hello/world. As the app container bind-mounts app/silex, you can edit e.g. app/silex/web/index.php and see the changes take effect immediately.